Industry Viewpoint: When the Maintenance Manual is Wrong

If you've been in maintenance for any length of time, you're bound to have run into this problem. The aircraft manufacturer's maintenance manual and the actual maintenance work that needs to be done are not in synch. This may be an occasional problem but even occasional problems can be fatal under the right conditions. This issue was driven home for me by a fatal air carrier accident involving rigging of aircraft flight control cables. I was involved in this accident investigation as an NTSB member.

The accident involved an aircraft that crashed shortly after mechanics had rigged the aircraft. The aircraft was destroyed on impact, killing the two crewmembers on board. Based on analysis of available information, including analysis of the flight data recorders, radar tracks, and eyewitness accounts, it was determined that the pilots were fighting for control of the aircraft and that the accident was caused by faulty flight control rigging.

Accident investigation
Determining what happened is only one step in accident investigation. The next step is why. Why were the flight control cables improperly rigged? Every mechanic is aware of the nightmare scenarios that happen — just as it happened here — when aircraft are misrigged. We all have it drilled into us to be extra vigilant when working with flight-critical cables.

To determine the why of the improperly rigged aircraft, we interviewed the mechanics involved, reviewed the relevant manuals and maintenance log pages, reviewed the facilities where the maintenance was performed, and the equipment actually used to perform the cable rigging and reconstructed how the maintenance was performed.

What we determined from this accident investigation is that the maintenance manuals were incorrect and failed to provide the guidance necessary for the mechanics to do their jobs properly. In this case, the mechanics rigged the elevator trim cables in accordance with the illustration in the manual which had the rigging backwards. This meant that when the pilots on the ill-fated flight took off and applied nose-up trim, the aircraft started to nose over. As they pulled back on the yoke to correct the nose down condition, it got more and more difficult to control the aircraft as the aircraft accelerated. The pilots applied more nose-up trim which, because the cables were rigged backwards, actually pushed the nose further down. With the aircraft so close to the ground and things happening so fast, the pilots never had a chance to figure out what was happening to their aircraft before it crashed.

Incorrect manual
In this case, we found that while the written manual instructions were technically correct, they were confusingly written and difficult to follow, so the mechanics relied on the illustration and basically ignored the text. What I found hard to understand in the course of this investigation, was why this problem hadn't been raised and fixed before. This particular manual, with its incorrect illustration, was around for decades. It seems to me that mechanics up until this ill-fated flight, had figured out that the illustration was wrong and properly rigged the aircraft — but never got the manual changed.

I know from experience as a mechanic and as an accident investigator, both for USAirways' mechanic's union (IAM) and the NTSB, that incorrect manufacturers' maintenance manuals are a problem and that mechanics usually work around them, without getting them corrected. This accident demonstrates the importance of mechanics raising to management problems they uncover with these manuals and for airline and repair station managers to push the manufacturers to correct the manuals.